This invention relates to a sintered body of ceramics composed primarily of silicon nitride and preparation thereof. More particularly, it pertains to a sintered body of ceramics which is high in density, excellent in mechanical strength and impact resistance and also low in the degree of lowering of mechanical strength even under an oxidizing atmosphere for prolonged time at a temperature of about 900.degree. C., and to a process for producing the same.
A sintered body of ceramics, which is excellent in thermal properties and has high density, is now attracting attention as a frontier material among various structural materials widely in various fields of industries. As a typical example, there is a sintered body of silicon nitride.
In the art of production of silicon nitride sintered bodies, there have generally been adopted the reaction sintering method, the hot press method and the ordinary sintering method.
Among them, the reaction sintering method is a method in which metallic silicon powders are molded previously into a desired shape, which is in turn heated gradually in an atmosphere of nitrogen or ammonia gas thereby to be converted into a nitride simultaneously with sintering (see, for example, Proc. of DARPA/NAVSEA, Ceramic Gas Turbine Demonstration Engine Program Review, ed. by W. Fairbanks and R. W. Rice, MCIC Report, March 1978 (MCIC-78-36)).
The hot press method comprises adding a sintering aid (e.g., Y.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3) to the powders of silicon nitride (Si.sub.3 N.sub.4), and sintering the resultant mixture in a certain mold (e.g., a mold of graphite) at a high temperature of 1700.degree. to 1800.degree. C. by application of a pressure of 500 kg/cm.sup.2 (see, for example, Norton Catalog.). According to this method, there can be obtained a sintered product having a high density with greater mechanical strength and also having excellent impact resistance as well as excellent thermal properties with a small degree of lowering of mechanical strength under an oxidizing atmosphere at higher temperatures. However, while this method involves the drawback of difficult fabrication of a complicated and large size sintered body on one hand, it is also disadvantageously inferior in capability of bulk production.
On the other hand, the ordinary sintering method comprises previously molding Si.sub.3 N.sub.4 powders and a sintering aid together with a binder such as paraffin, and then sintering by heating the compact as such without hot press under a non-oxidative atmosphere (see, for example, GTE Sylvania Catalog. or Carborundum Co. Technical Sheet). According to this method, however, it is difficult to obtain a sintered body having a high density with excellent mechanical strength and impact resistance.
In view of the state of the art as outlined above, the present inventors have made various investigations on the above ordinary sintering method and consequently have proposed an improved ordinary method comparable with the hot press method, which is capable of producing a high density sintered body excellent in mechanical strength and impact resistance (Japanese Provisional Patent Publication Nos. 113674/1980 and 116670/1980).
However, the silicon nitride sintered bodies obtained according to these methods still proved to be unsatisfactory in resistance to lowering of mechanical strength under a high temperature oxidizing atmosphere.
The present inventors have made further extensive studies on the above point and at last accomplished the present invention.